The purpose of this article is to explain pointers of course. There isn't much good information out there, and tons of confused people ask me about them, so I thought I'd write this up. A pointer is simply a variable in C or C++ which has an asterisk before it. An example would be something like this:
void *Pointer;
What a pointer does, simply put, is point to a memory location. All pointers, no matter what data type you use for them, whether it be void, int, long, char, or whatever, are 4 bytes in size. These 4 bytes contain the memory location pointed to. In DOS, with _far pointers, the first 2 bytes are the offset address. The last 2 bytes contain the segment address. In 32bit operating systems, such as Windows 95/95/NT/2000, also known in programming as Win32, all 4 bytes together contain the address, since 32bit memory is linear. There are no offsets or segments in 32bit memory. Here is an example of a DOS function to return a pointer to the specified memory address:
void _far *MakeFarPointer(unsigned short OFFSET, unsigned short SEGMENT)
{
union tagUNION
{
struct tagADDRESS
{
unsigned short OFFSET, SEGMENT;
} ADDRESS;
void _far *POINTER;
} UNION;
UNION.ADDRESS.OFFSET = OFFSET;
UNION.ADDRESS.SEGMENT = SEGMENT;
return POINTER;
}
Now in a lot and most DOS compilers, you can simply have a pointer equal the numeric value of the address, like you can in Win32. The early Microsoft Visual C++ compilers that supported DOS didn't do well with doing that though. It pointed at the number, instead of pointing to where the number says to go. I play it safe using the above function. To point a pointer at a location using this function, you would do the following:
unsigned long _far *somepointer = (unsigned long _far *)MakeFarPointer(0xA000, 0x0000);
In Win32, with the 32bit memory, it is very simple to point to a location:
unsigned long *somepointer = (unsigned long *)0xA389B036;
Now, you've got a pointer of type unsigned long. The best way to now treat this as a non-pointer, and place data in it, is to use the array brackets. Just do "somepointer[0] = number;", or otherwise anything you have it equal becomes its new address it points to. Using array brackets is the easiest way to avoid doing that.
So really, a pointer is just a way to choose where a variable is stored. Here is some code to look at:
int variable = 10;
int *somepointer = &variable
This makes somepointer point to the address of variable. The & symbol means "address of". Now look at the following:
somepointer[0] = 25;
This means variable will no longer be 10, but is now 25. Get it? If not, then I guess I suck at writing articles.
